1. Field of the Invention
The present invention relates to a solid-state memory device, and more particularly to a solid-state memory device using a superlattice device. The present invention also relates to a data processing system and a data processing device that include such a solid-state memory device.
2. Description of Related Art
In recent years, a semiconductor memory called PRAM (Phase change Random Access Memory) has been commanding attention. The PRAM is a semiconductor memory device using a phase change material as a material of a recording layer, and records therein information by using a difference between an electric resistance in a crystal phase and an electric resistance in an amorphous phase.
Specifically, when a chalcogenide compound is used for a phase change compound, an electric resistance becomes relatively low in a crystal phase, and an electric resistance becomes relatively high in an amorphous phase. Therefore, stored data can be read out as the electric resistance of a phase change compound is detected by passing a read current. Regarding data writing, a phase of a phase change compound can be changed to a crystal phase when the phase change compound is heated at or higher than a crystallization temperature and lower than a melting point during a certain period of time or more by passing a write current. On the other hand, the phase of the phase change compound can be changed to an amorphous phase when the phase change compound is heated at or higher than a melting point by passing a write current and is rapidly cooled thereafter.
However, a relatively large energy is necessary to change a phase of the phase change compound between an amorphous state and a crystal state. Therefore, a conventional PRAM has a problem that a large current is necessary at the time of rewriting data. To solve this problem, U.S. Patent Application Publication No. 2004/0188735 describes a technique of reducing a write current necessary to change a phase by configuring a recording layer to have a phase-change material layer and a high-resistance-phase-change material layer laminated alternately.
However, according to the technique of U.S. Patent Application Publication No. 2004/0188735, it is also difficult to greatly reduce the current required to change a phase because a phase of a part of the recording layer is changed between an amorphous state and a crystal state. Also, this conventional technique does not solve a problem of slow operation speed, because a relatively long time is required to change between an amorphous state and a crystal state.
Furthermore, according to the technique of U.S. Patent Application Publication No. 2004/0188735, a total phase of a part of the alternately laminated phase-change material layer and the high-resistance-phase change material layer (“region A” in the patent document) is changed. Therefore, repeated rewriting of data causes mixing of these materials. Such a change in the film quality of the recording layer degrades its characteristic and also reduces the number of rewritable times.
Basically, when a phase of a phase change material is changed between an amorphous state and a crystal state, it is difficult to control any of the states in a uniform state. For example, in a crystal state, there is generated many regions having crystal defects and disordered crystal-lattice arrangements, as well as crystal boundary regions or the like, and these states change each time when a phase changes. In this state, when a current is passed by applying a voltage to an electrode, the current flows in regions having crystal defects and disordered crystal-lattice arrangements, and in the crystal boundary regions. As a result, a current becomes uneven, and a current path changes each time when a phase is changed. Consequently, a resistance varies each time when a phase is changed, and this causes a problem such that data cannot be read correctly.
The present invention has been achieved to solve such problems.